The invention relates to a process gas for use in laser machining processes, such as laser welding or laser beam fusion cutting. The invention further relates to a process for laser machining materials, whereby a focused laser beam impinges against the surface of the work piece to be machined, and a process gas stream is directed against the surface of the work piece.
Due to the properties of the laser beam, in particular its intensity and ease of focusing, lasers are used today in many areas of material processing. The laser machining systems used are known in the art. In general, these comprise a laser machining head, if applicable comprising a nozzle arranged coaxially to the laser beam. Often, laser machining systems are used in conjunction with CNC controls of guide machines for an x-y cutting direction. In addition, in laser beam cutting, handling systems for three-dimensional work pieces are being used with increasing frequency. An automatic adjustment of cutting parameters (laser power adjusted to the current cutting speed during the cutting process) based upon the contour to be cut is generally a prerequisite for a good cut quality, even around sharp corners and acute angles.
Within the scope of the invention, a focused laser beam is understood to be a laser beam that is focused essentially on the surface of the work piece. In addition to the methods predominantly used, comprising laser beams that are focused on the surface of the work piece, the invention can also be applied with the seldom used variant in which the beam is focused not precisely on the surface of the work piece.
In many laser material machining processes, metallic and/or other materials are heated to temperatures at which a reaction with the enveloping gases occurs. Thus, in many cases industrial gases are employed in order to allow these material machining processes to be implemented more effectively, more rapidly, and/or with improved quality.
Worldwide, laser beam cutting is the most frequently employed laser machining process. For example, in Germany more than 80% of laser machining systems are used for cutting. In laser beam cutting, differentiation is made between laser beam flame cutting, laser beam fusion cutting, and laser beam sublimation cutting. In laser beam fusion cutting, the material is melted open at the point of separation using the laser beam. The melted material is forced out of the cut joint via a process gas. Laser beam fusion cutting using a process gas under high pressure has proven effective in cutting specialty steels, but is also used with other materials such as structural steels and aluminum. An inert gas is usually used as the process gas in laser beam fusion cutting.
In laser beam welding, process gases fulfill different purposes. The control and reduction of the plasma is imperative at high laser power levels. This is known, for example, from the publication “Laser im Nebel” [Laser in the Mist], Dr. W. Danzer and Klaus Behler, journal LASER, edition 1/87, pages 32 through 36. Other objectives such as protection against oxidation, metallurgical optimization, and/or a maximization of speed and quality (spatter formation, pores, seam quality) have up to now been neglected. In laser beam welding, the process of using inert shielding gases such as helium or argon is known. Nitrogen is also used at times. Now and then, admixtures are also added in small quantities.
The speed of the welding and cutting processes with the laser beam is limited by the balance between “energy introduced—energy lost (radiation, thermal conduction)”. The energy of the laser beam is highly concentrated, however with materials that cannot be cut with the help of the exothermal reaction with oxygen this energy becomes the limiting parameter.
For example, in cutting a 3 mm steel sheet using a 900 w laser and pure oxygen as the process gas, in addition to the 900 watts of energy coming from the laser, an additional 600 watts from the burning of the iron in the cutting joint enter into the cutting process. This results in a cutting speed of approximately 3 m/min.
However, in cutting a 3 mm chromium-nickel steel sheet using a 900 watt laser, for example, which due to the resulting slag is not cut with oxygen, but must be cut with an inert gas such as nitrogen or argon, this additional energy is missing from the reaction Fe+½O2→FeO. The maximum cutting speed decreases correspondingly to approximately 1.5 m/min.
An object of the present invention is thus to reveal a process gas and a method of the type described at the beginning which will permit improved laser machining. In this, a high cutting speed for laser beam fusion cutting is sought. In particular, the goal is to enable a high-quality, process-safe, and reproducible laser beam fusion cutting process. In laser beam welding, the goal is to achieve a maximization of speed and quality, in addition to controlling and reducing the plasma, using the process gas specified in the invention. In this, the invention is aimed primarily at cases in which an inert gas is customarily used as the process gas.
This object is attained according to the invention with a process gas that contains at least oxygen and hydrogen, in addition to at least one inert gas.